This invention provides a method for adaptive radiation therapy (ART) in which the location of target tissue is detected in a digital radiograph in order to insure proper targeting of therapeutic radiation.
Many improvements in radiation therapy have the purpose of delivering therapeutic radiation to a target (such as a cancerous tumor) while minimizing exposure to normal tissue. These improvements allow a greater dose of radiation to be applied to the tumor with the constraint that the dose received by surrounding normal tissue must be limited.
Planning for radiation therapy starts with obtaining a three-dimensional image of the patient while the patient has two or more external markers attached. The imaging modality allows the physician to precisely identify the boundaries of the tumor. Computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound can be used for this purpose.
The volume of the tumor as it appears in the image is generally referred to as the gross tumor volume (GTV). The GTV is expanded to take into account microscopic extensions of the tumor. This expanded volume is typically referred to as the clinical tumor volume (CTV). The CTV can be further expanded because of potential setup error in the treatment phase. In the case of extra-cranial tumors, there is also uncertainty in tumor position relative to the external markers due to organ motion. For example, lung tumors move as the patient respires. The expansion of the CTV to compensate for setup error and uncertainties due to organ motion is often referred to as the planned treatment volume (PTV).
During setup for radiation therapy, the patient is positioned so that the PTV is located at the system's isocenter. In order to correctly position the patient, the system detects the position of the external markers. Since the position of the PTV is know relative to these external markers, the system can move the patient into the proper position.
In intensity modulated radiation therapy (IMRT), the therapeutic beam sweeps out an arc about the isocenter so that the PTV receives radiation for the duration of the treatment while other tissue is irradiated for a fraction of the time. As the beam moves, its shape is periodically adjusted by means of a multileaf collimator (MLC) to conform to the shape of the PTV from the perspective of the therapeutic radiation beam. In order to further spare normal tissue, the full dose is given over a number of fractionated treatments. Fractionated treatments usually comprise 20 to 40 partial doses given over a period of several days to several weeks.
The PTV is larger than the CTV because of uncertainty in the location of the target relative to the isocenter that needs to receive the full dose of therapeutic radiation. One source of uncertainty is that the tumor may move relative to the external markers between the time of imaging in the planning phase and setup in the therapeutic phase. Furthermore, since the dose is usually given in fractionated treatments the position of the target may vary relative to the external markers, internal organs, and the isocenter differently at each treatment.
A number of methods have been developed to reduce the uncertainty in the location of the target with respect to the system's isocenter. For example, if organ motion due to respiration is a cause of uncertainty, then it can be reduced by capturing the planning images and performing treatment in a specific respiratory state such as relaxed expiration.
Radiation therapy systems are sometimes equipped with two digital radiography units to obtain stereoscopic x-ray images prior to treatment. These images are compared with digitally reconstructed radiographs (DRR) from the CT images captured in the planning phase. Registration of bone or implanted metal markers in the radiographs and DRRs is used to adjust the position of the patient so that the PTV is at the isocenter.
Electronic portal imaging can be used to confirm the location of the target. In electronic portal imaging, the therapeutic beam is imaged after it passes through the patient. This image can be acquired during radiation therapy or prior to therapy with the therapeutic beam source set to low intensity. A drawback of this method is that therapeutic radiation is generally above 1 MV in photon energy, and consequently has low soft tissue contrast. Also, portal imaging is limited to a single radiation source which can only locate the target in two dimensions at an instance in time. This limitation can be overcome by using collecting portal images at several angles and performing volumetric reconstruction as described by E. C. Ford et al. in “Cone-Beam CT with Megavoltage beams and an amorphous silicon electronic portal imaging device: Potential for Verification of Radiotherapy of Lung Cancer,” Med. Phys., Vol. 29, No. 12, pp. 2913-2924 (2002). However, a disadvantage of this method is that target position verification results in significant radiation dose to the patient. Also, with current technology, the time required to verify the target's position is too long to ensure that the target has not moved in the time taken to verify its position.
US Patent Application No. 2004/0158146 (Mate) is directed to a guided radiation therapy system having implanted markers that are excitable by an external radiation source. The implanted markers are imaged so that their position relative to the target is known. During patient setup for radiation treatment, the position of the internal markers are located by a sensor array external to the body. Based on the position of the internal markers as determined by the sensor array, the patient is positioned so that the target is at the isocenter.
U.S. Pat. No. 6,501,981 B1 (Schweilkard) is directed to a method to track an internal target in the presence of respiratory motion. Internal markers are placed near the target. Before treatment, the position of the internal and external markers is imaged as the patient breaths. Based on this image data, a correlation between the position of the internal and external markers is calculated. When the patient is treated, the position of the target is predicted by continuously monitoring the position of the external markers. Periodically, the internal markers are imaged in order to obtain their actual location.
Shinichi et al. in “Detection of Lung Tumor Movement in Real-Time Tumor-Tracking Radiotherapy,” Int. J. Radiation Oncology Biol. Phys., Vol. 51, No. 2, pp 304-310 (2001) describes a system for real-time tracking of internal 2.0 millimeter gold markers in three dimensions. Four sets or diagnostic fluoroscopes were used to image the markers. During therapy the target was only irradiated when the marker was detected within a permitted dislocation from a nominal location.
A shortcoming of current methods of radiation therapy is that the clinical tumor volume (CTV) is expanded to include surrounding space in order to compensate for uncertainty in location of the target relative to the isocenter. As a result, normal tissue receives a damaging dose of radiation.
Methods have been developed that use implanted internal markers that reduce target location uncertainty. Unfortunately, marker implantation requires addition surgery and may not be an option if the tumor location is inaccessible or if too many tumors are present. Also, the position of an internal marker may not be perfectly correlated with the position of the target.
A feature of the present invention is to provide a system in which the location of the target can be determined accurately. Another feature of the present invention is to provide a system that does not employ internal markers for target location. Another feature of the present invention is to provide a system in which the location of the target can be determined quickly and without significant additional radiation exposure to normal tissue.